1. Technical Field
The present disclosure relates to a laser generation device and laser light source device. More specifically, the present disclosure relates to a laser generation device and laser light source device for producing a continuous-wave laser of deep ultraviolet (deep-UV, or DUV) at a wavelength around 213 nm by using nonlinear wavelength conversion techniques.
2. Description of the Related Art
Laser sources using wavelength conversion by nonlinear materials are now widely deployed into industry; however, they still suffer from reliability issues. Limiting factors to the reliability include damage and/or degradation in nonlinear optical crystal elements used for wavelength conversion. The damage/degradation comes from small spot size of the laser light focused onto the crystal; it follows that, even a minor damage/degradation can cause significant decrease in efficiency because the conversion takes place in an optical resonator. When the process uses light at infrared to visible range for the fundamental wave for generating the ultraviolet light, damage caused by high photon energy of the ultraviolet light is significant.
Damages in wavelength conversion elements are usually caused by the ultraviolet light, which gives rise to optical loss for the fundamental light to be used as input to the elements. This phenomenon would be understood from the fact that the photon energy increases in an inversely proportional manner to the wavelength in the ultraviolet light and therefore the increased energy should have more significant impact on the material, and the fact that, the shorter the wavelength is, the more significant optical absorption becomes in most optical materials. Although the absorbed energy can be finally dissipated away in a form of heat, the impact onto the material during the process should be noticeable, and in most cases the optical elements suffer from degradation over time and result in poor performance. This impact is particularly significant in continuous-wave light sources, where the wavelength conversion is performed in optical resonators.
The continuous-wave deep ultraviolet (DUV) laser light can be obtained through a nonlinear wavelength conversion, in which the conversion efficiency is proportional to input power. A typical coefficient of the proportionality, or “normalized conversion efficiency,” is of the order of 10−4 W−1, suggesting that 100 W is required for the input in order to obtain 1 W for the output. Therefore it is particularly necessary for having practical conversion efficiency to dispose a nonlinear optical crystal, or a wavelength conversion element, into an optical resonator, thereby increasing light intensity of the input power at the element. Output power of 2 W at a wavelength of 266 nm has been achieved for a laser of continuous-wave operation when a laser of near-infrared range at a wavelength of 1.064 μm (1064 nm) is adopted for a fundamental wave, wherein the wavelength of 266 nm corresponds to the fourth harmonic of the fundamental, that is, a second harmonic of a wavelength of 532 nm, which is a second harmonic of the fundamental. Currently, the degradation of the nonlinear optical crystal is not negligible for such a high operation power as 2 W; therefore the degradation is a limiting factor to device life.
Industrial applications, such as inspection on semiconductor wafers or reticles, call for laser sources of shorter wavelength with higher output power. Among others, a laser source to generate light at a wavelength of 213 nm, the fifth harmonic of a near-infrared laser source of a wavelength of 1064 nm mentioned above, has been proposed as a promising candidate for inspection applications in next generation semiconductor industry, and numerous experiments have been reported. They adopt a sum-frequency mixing (SFM) process for producing the fifth harmonic from the fourth harmonic at a wavelength of 266 nm and the fundamental at a wavelength of 1064 nm. For experiments of pulsed sources there are a few reports based on SFM between the second harmonic and the third harmonic; however, when it comes to continuous-wave demonstrations there are only reports using the fourth harmonic and the fundamental for SFM. There is a report of demonstration of a continuous-wave (CW) 213 nm wavelength laser with output over 100 mW (Non-Patent Literature 1). This report discloses that, while an external resonator is kept at resonance by fundamental light at a wavelength of 1064 nm, a fourth harmonic at a wavelength of 266 nm is incident with tight focus into a nonlinear crystal, where the beam of 266 nm overlaps with that of 1064 nm resonator mode.
However, in such apparatus, the laser light at a wavelength of 266 nm will have very high optical power density (intensity) in the wavelength conversion elements. Because requests from the industry for such laser sources call for the output powers of the order of 1 W with continuous-wave operation at 213 nm, at least several watts of 266 nm light should be focused tightly onto the wavelength conversion element. This gives rise to serious concern on the degradation of the elements caused by such strong intensity of the ultraviolet light not only of the fifth harmonic generated, but also of the fourth harmonic focused tightly.